MECHANISM OF THE TRANSFORMATION OF PRESSURE FLUCTUATION PRODUCED BY KARMAN VORTEX SHEDDING INTO NOISE

通过理论和实验研究发现，叶片、NACA 0008或NACA 0015置于均匀迎面流中、以及旋转平板的近尾流中，产生的噪声与近尾流速度脉动特性密切相关。结果总结如下：(1)在较厚的叶片NACA 0015的情况下，如果攻角θ小于约5度，则会产生离散频率噪声。在这种情况下，压力侧的流动在叶片后缘的上游分离，并产生卡门涡街，这是产生离散频率噪声的原因。 (2)对于较薄的叶片NACA 0008，当theta=0时，不会出现DFN，因为没有逆压梯度强烈的位置。然而，如果存在一定的攻角，即使攻角很小，由于其相对尖锐的几何形状，流体也会在叶片前缘附近分离。然后离散频率噪声产生。 (3)在NACA 0008叶片的情况下，实际上距离叶片后缘较远的地方会产生周期性速度脉动，但不会出现DFN。其原因是气流附着在叶片后缘上。这一事实清楚地表明，DFN产生的必要条件是近尾流中存在较大的死流区域。 (4)尽管迎面流与旋转流场一样沿跨度方向发生剪切，但仍会发生DFN。这意味着DFN的生成机制与均匀主流中的生成机制基本相同。 (5) 旋转平板产生的噪声频谱的宽带性质可以通过物理模型进行理论上解释，其中音调噪声是由每次卡门涡旋脱落导致的周期性速度脉动局部产生的，并且它们在叶片跨度方向上积分。

It is theoretically and experimentally investigated that the generated noise is closely related with the characteristics of the velocity fluctuation in the near wakes of a blade,NACA 0008 or NACA 0015 placed in a uniform oncoming flow,and in the near wake of a rotating flat plate. The results are summarized as follows:(1)In the case of a thicker blade,NACA 0015,the discrete frequency noise is generated if the attack angle theta is less than about 5 degree. The flow on the pressure side, separates in this case just upstream of the trailing edge of the blade and the Karman vortex street generates,which is the cause to generate the discrete frequency noise. (2)In the case of a thinner blade,NACA 0008,the DFN does not occur if theta=0,because there is no location where adverse pressure gradient is intense. However if there is some attack angle,even if it is small,the flow separates near the leading edge of the blade due to its relatively sharp geometry. Then discrete frequency noise generates. (3)In the case of NACA 0008 blade periodic velocity fluctuation is actually generated relatively far from the blade trailing edge,the DFN,however,does not occur. The reason of this is the flow attached up to the blade trailing edge. This fact shows clearly that the necessary condition of the generation of DFN is the existence of a large dead flow region in the near wake. (4)Although the oncoming flow is sheared in the span direction as in the case of the rotating flow field,DFN occurs. This means the mechanism of the generation of DFN is fundamentally the same that in the uniform main flow. (5)The broad band nature of the spectrum of noise generated from the rotating flat plate can be theoretically explained by the physical model in which the tonal noises are generated locally by the periodic velocity fluctuations due to each Karman vortex shedding and they are integrated in the blade span direction.